A high fever, generally defined as a body temperature of 103°F (39.4°C) or above in adults, is most often caused by your body fighting an infection. But infections aren’t the only trigger. Medications, autoimmune conditions, heat exposure, and even certain cancers can push your temperature into a high range. Understanding the specific cause matters because it determines whether the fever will resolve on its own or needs urgent treatment.
How Your Body Creates a Fever
A fever isn’t a malfunction. It’s a deliberate response controlled by a small region in your brain that acts as your internal thermostat. When your immune system detects a threat, such as bacteria or a virus, immune cells release signaling molecules called cytokines. These signals reach the brain through two routes: directly through the bloodstream, or indirectly by stimulating nerve fibers that relay the message.
Either way, the end result is the same. Your brain’s thermostat produces a chemical messenger that raises the body’s temperature set point, much like turning up a dial on a thermostat at home. Your body then works to reach that new, higher target. Blood vessels near your skin constrict to conserve heat, your muscles may shiver to generate it, and you feel cold even though your temperature is climbing. Once the threat passes and those chemical signals fade, the set point drops back to normal and you start sweating to cool down.
Infections: The Most Common Cause
The vast majority of high fevers trace back to an infection. Viruses like influenza, COVID-19, and respiratory syncytial virus (RSV) are frequent culprits, especially during colder months. Bacterial infections, including strep throat, urinary tract infections, pneumonia, and ear infections, also commonly produce fevers above 103°F. Fungal and parasitic infections are less common in developed countries but can cause prolonged fevers in people with weakened immune systems or those who have traveled to certain regions.
The height of the fever doesn’t always correlate with how serious the infection is. Some mild viral illnesses can spike a fever to 104°F, while serious bacterial infections occasionally produce only a low-grade fever, particularly in older adults or people with compromised immune systems. What matters more than the number on the thermometer is the pattern of the fever and the symptoms that accompany it.
Non-Infectious Causes
When a fever keeps returning or persists without an obvious infection, the cause often lies elsewhere. Autoimmune conditions like lupus and rheumatoid arthritis can trigger fevers during flares, when the immune system mistakenly attacks healthy tissue. A separate group of conditions called autoinflammatory diseases cause recurring fevers due to problems with the body’s innate immune response rather than an acquired one. These include familial Mediterranean fever and adult-onset Still’s disease, among others. In children, a condition called PFAPA (periodic fever with mouth sores, sore throat, and swollen glands) is one of the more common autoinflammatory causes of recurring high fevers.
Certain cancers, particularly lymphomas and leukemias, can produce persistent or cyclical fevers. These fevers often come with drenching night sweats and unexplained weight loss. Inflammatory conditions that don’t involve the immune system attacking itself, such as deep vein blood clots or tissue damage after surgery, can also raise body temperature significantly.
Medications That Can Trigger Fever
Drug-induced fever is more common than many people realize, and it’s easy to overlook because it mimics infection. Several classes of medications are known to cause fevers as a side effect or allergic-type reaction. Anticonvulsants, certain antibiotics, blood thinners like heparin, the gout medication allopurinol, and newer cancer immunotherapy drugs are among the most frequently implicated.
Some medication reactions produce dangerously high fevers through specific syndromes. Malignant hyperthermia is a rare, life-threatening reaction to certain anesthesia drugs that can send body temperature soaring above 104°F during surgery. Neuroleptic malignant syndrome, triggered by antipsychotic medications, causes extreme fever along with muscle rigidity and confusion. Serotonin syndrome, which can occur when medications that boost serotonin levels overlap or are taken in excess, produces a similar picture. All three of these are medical emergencies.
Fever Thresholds by Age
What counts as a “fever” depends partly on where you measure it. A rectal, ear, or forehead temperature of 100.4°F (38°C) or higher is considered a fever for all ages. An armpit reading of 99°F (37.2°C) or higher qualifies. Adults with temperatures of 103°F or above will usually look and feel noticeably sick.
In infants and young children, fever thresholds carry more weight. Any fever of 100.4°F or higher in a baby under three months old is treated seriously because young infants can’t fight infections as effectively and may deteriorate quickly. Children between six months and five years old face a specific risk: febrile seizures. These seizures are triggered by fever, not by an underlying brain problem, and while they’re frightening to witness, most don’t cause lasting harm. A family history of febrile seizures, young age (under 18 months), and rapid onset of fever all increase the likelihood.
Fever of Unknown Origin
Sometimes a high fever persists for weeks without a clear explanation despite initial testing. Doctors classify this as a fever of unknown origin when the temperature reaches at least 101°F (38.3°C) on multiple occasions and standard workups haven’t identified a cause. The diagnostic process for these cases is methodical and layered: blood cultures, urine cultures, chest imaging, and blood tests for inflammation markers come first. If those don’t reveal anything, testing expands to include screenings for specific viruses (Epstein-Barr, cytomegalovirus, hepatitis, HIV), tuberculosis, and imaging of the abdomen and pelvis to look for hidden abscesses or tumors.
When all noninvasive tests come back empty, a tissue biopsy becomes the next step. Depending on the clinical picture, this could involve sampling tissue from the liver, a lymph node, bone marrow, or skin. The three most common categories that eventually explain a fever of unknown origin are infections (especially tuberculosis and abscesses), cancers, and autoimmune or inflammatory conditions.
Warning Signs Alongside a High Fever
A high fever alone isn’t always an emergency, but certain accompanying symptoms change the picture entirely. A stiff neck combined with a severe headache, high fever, and confusion is the classic warning pattern for meningitis, an infection of the membranes surrounding the brain and spinal cord. Vomiting, seizures, sensitivity to light, and a skin rash can also appear. In infants, the signs look different: constant crying, extreme sleepiness, poor feeding, limpness, and a bulging soft spot on the head.
Sepsis, a body-wide response to infection that can cause organ failure, also begins with fever. Rapid heart rate, rapid breathing, confusion, and skin that looks mottled or feels cold and clammy despite a high temperature are warning signs. A fever paired with severe abdominal pain could point to appendicitis or another abdominal emergency. In all of these situations, the fever itself isn’t the primary danger. It’s the underlying condition driving it that requires immediate attention.